Conventionally, when forming a floating gate (to be referred to as an FG hereinafter) electrode made of p-type polysilicon of a nonvolatile semiconductor memory device, a p-type impurity such as boron is doped by ion implantation or plasma doping. Even when the p-type impurity is doped in a shallow portion of the FG film, the p-type impurity diffuses through an annealing step to be performed later, so the p-type impurity concentration distribution in the FG film becomes uniform.
The operation of an FG cell formed as described above poses the following problem. If the concentration of the p-type impurity is relatively low, the FG upper portion may be depleted during a write operation, and this may worsen the write operation. On the other hand, if the concentration of the p-type impurity is relatively high, electrons necessary for erase may not be supplied to the FG lower portion during an erase operation, and this may make the erase operation impossible. As described above, the conventional method in which the p-type impurity such as boron uniformly distributes in the FG has the problem that it is difficult to reliably perform both a write operation and erase operation.